Synthetic materials are an indispensable part of people's daily life. With a wide variety of products, from consumer goods to industrial equipments, from automotive parts to electronic devices, they have played important roles due to their wide range of properties. However, in many cases, a particular application may demand certain performance(s) that the synthetic materials alone, such as synthetic polymers, cannot offer. Yet the enhanced characteristics or performances can be achieved with certain composites, which are formed by combining synthetic resins with one or more components with significantly different physical or chemical properties. In a polymer composite, also frequently referred to as a polymer matrix composite (PMC), the polymer constitutes a continuous phase, while the other components are embedded in the phase to serve as fillers or reinforcements. The most widely used polymer resins include (unsaturated) polyesters, epoxy, phenolics, vinyl esters, polyurethanes, and polyimides. The common reinforcements are glass fibers, carbon fibers, aramid fibers and boron fibers. Sometimes fillers and reinforcements are modified through surface treatments so as to improve their wettability with polymer resins.
Moreover, the combination of organic and inorganic components at nanometer-scale or molecular level leads to a new category of composite materials termed organic-inorganic hybrid materials. Depending on interaction connecting organic and inorganic components, organic-inorganic hybrid materials are classified as either Class I materials in which the components interact weakly through hydrogen bonding, van der Waals force or electrostatic attraction, or Class II materials in which the organic and inorganic species are linked through stronger chemical bonding such as ionic or covalent bonds. Organic-inorganic hybrid materials were desired as specific mechanical, optical, thermal, electronic, magnetic, dielectric or other properties can be incorporated in the materials along with the inorganic or inorganic-like components.
Many inorganic components in organic-inorganic hybrid materials, especially certain silicon oxides, siloxanes and metal oxides, have been prepared through sol-gel technique. This widely used process starts with a colloidal solution (sol) of organosilanes, and/or metal alkoxides and/or metal salts, and undergoes a series of hydrolysis and condensation reactions, to yield gel-like integrated network as desirable for a particular application. The sol-gel process is a wet-chemical process that is catalyzed by either a base or an acid. Water is supplied as a reactant to participate in the hydrolysis reaction, and depending on the amount of water introduced; hydrolysis and condensation reactions can be reached at various levels. The outcome of the product composition, morphology and viscosity can be quite different as well. The chemistry, process, products and physics of sol-gel technique have been well documented in many academic books and scientific publications. However, several intrinsic issues limit the application scope of materials prepared by sol-gel process. The extents of the hydrolysis and condensation reactions are hard to control consistently, often giving rise to unrepeatable composition and viscosity. Aside from generating a large amount of organic solvents and alcohols, the sol-gel chemistry usually suffers low yields due to incomplete hydrolysis and condensation. In the firing process that follows the wet chemistry, high temperature is needed to drive out water. Water removal through capillary pressure can cause extremely high internal stress. Crack formation and stress evolution lead to defects in the fabricated products.
Of extensive interest as components in the organic-inorganic hybrid materials are the silsesquioxane compounds expressed by the general formula (RSiO1.5)n. Structurally these silsesquioxanes can be random polymer networks, ladder polymers, incompletely condensed polyhedral frameworks, and fully condensed polyhedral cages, as often referred to as polyhedral oligosilsesquioxanes, or POSS. A convenient denotation T is adapted to represent silicon atom with three siloxane oxygen atoms attached to it. Although smaller POSS compounds such as T4 and T6 species were synthesized, the preparation was not easy and usually led to larger structures. On the other hand, larger POSS compounds such as T10 and T12 were made, but they are rare and are usually synthesized in low yield. The eight-silicon structure T8 species have received most attention. They are relatively easy in their syntheses and the symmetrical cage structure brings a number of desirable properties including chemical stability, thermal stability, dimensional stability, abrasion resistance and viscoelastic properties.
POSS compounds can be synthesized by hydrolysis and condensation from trifunctional precursors RSiX3, where R is either a hydrogen atom or a hydrocarbon group, sometimes bearing certain functionality that can be utilized for further reactions in an intended application, and X is a hydrolysable moiety, such as a chlorine atom, an alkoxyl group, or a silanol group. Other than coming directly from its precursors, substitute groups on POSS compounds may be “attached” by functionalization reactions as well. In order to achieve certain enhanced properties, metal atoms were brought into POSS structure as doping agent. For example, titanium atoms were included to partially replace some silicon atoms. The new material with titanium atoms as dopant showed improved refractive index for optical applications. In an organic-inorganic hybrid material composition, POSS compounds can be linked to the organic medium by pendant reactive functional groups such as (meth)acrylate, epoxy, thiol-ene, styryl, oxetane, cinnamate, etc. Though POSS compounds exhibit many unique properties, their syntheses are quite difficult, time consuming and of low yields, and thus their applications are significantly limited by these drawbacks.
In view of the above, it would be desirable to provide a method for well-controlled, repeatable, fast and high-yield preparation of organic-inorganic hybrid material compositions, which can lead to polymer composites with certain properties such as desirable scratch resistance, abrasion resistance, stain resistance, thermal stability, dimensional stability, flame retardancy, oxygen barrier and moisture barrier properties, dielectric constants, or high refractive index.